It would be immensely useful for the management of Lyme disease (LD) treatment to have available a test for cure. Such a test could be employed not only to ascertain if treatment of acute LD was successful, thereby preventing the transition to the chronic, more intractable form of the disease, but also to distinguish among the possible etiologies of the so-called post-treatment LD syndrome. The PI and coworkers recently developed a sensitive and specific enzyme-linked immunosorbent assay (ELISA) for the serological diagnosis of LD. The test is based on the detection of antibody (Ab) to an immunodominant, invariable region (lR) of the lipoprotein VIsE. VIsE is the molecule that undergoes antigenic variation in Borrelia hurgdorfen (the etiologic agent of LD). A peptide (C6) representing the invariable region 6 (IR6) of VIsE serves as antigen. It is hypothesized that, because the spirochete should not simultaneously express on its surface more than one (or a few) VIsE variant(s) at any time, the VIsE lipoprotein must be rapidly turned over and degraded by the spirochete as new variants are progressively expressed. As a consequence of this postulated intrinsic instability, VIsE should be scarce on dead or dying spirochetes, and secondary Ab responses to the C6 peptide should decline in unison with the infection's demise, following antibiotic treatment. It is further hypothesized that the decline in titer of the C6 Ab as a function of time after treatment may serve as a test for Lyme disease cure. Preliminary results indicate that the C6 ELISA titer in cured patients falls by a factor greater or equal than 4 whereas for treatment-resistant patients the fall is by a factor <4. This is similar to the VDRL test used to diagnose syphilis cure.The broad, long-term objective of this project is to assess both retrospectively and prospectively the ability of the C6 ELISA to serve as a test for LD cure. In this proposal the C6 test will be assessed retrospectively by achieving three specific aims: Specific Aim 1: To assess retrospectively the C6 ELISA as a test for cure in patients with acute LD. Serial serum samples from patients with either erythema migrans ( n = 90) and/or culture-confirmed infection ( n = 156) will have been collected at presentation and at 6 and 12 months thereafter. The samples will be titrated for anti-C6 Ab. Specific Aim 2: To assess retrospectively the C6 ELISA as a test for cure in patients with chronic LD and post-treatment LD syndrome. Same as for SAl, but with patients with chronic LD (n = 150) and post-treatment LD syndrome (n = 60). Specific Aim 3: To assess the C6 ELISA as a test for cure in animal models of LD. Cure of LD will be assessed objectively (by culture and PCR) both in rhesus monkeys (chronic LD) and in mice (acute LD). Correlation between LD cure and anti-C6 Ab titers will be evaluated.